1. Field of the Invention
The present invention relates to a quantizer, and more particularly, to a noise shaping quantizer which has multi staged loops.
2. Description of the Related Art
It is well known that, in a digital signal processing, a quantizer is an important elemental technology. When the quantizer performs quantizing operation, quantization error is inevitably observed. Therefore, efforts and inventive approaches in reducing the quantization error at least in a frequency range including necessary signals have been made in recent years. As one of such approaches, a technique called noise shaping has been developed.
FIG. 1 is a block diagram of a multi stage noise shaping quantizer 101 having plural stages of loops, which is disclosed at Patent Reference 1 (Japanese patent gazette No. 8-2024-B2). The multi stage noise shaping quantizer 101 includes a main loop 103 which has two adders 111 and 113, a first local quantizer 117, a limiter 141, a subtractor 121, and a delay element 123. The main loop 103 functions as a single-integration noise shaping quantizer implementing a first order noise shaping. In addition, an adder 125, a second local quantizer 127, a subtractor 129, and a feedback circuit 131 constitute a sub-loop 105. The sub-loop 105 functions as a multi-integration noise shaping quantizer implementing a first, a second or higher order noise shaping.
A delay element 133 and a subtractor 135 constitute a differentiator 107. The differentiator 107 differentiates an output of the sub-loop 105 corresponding to the noise shaping order of the main loop 103. The output of the main loop 103 and the output of the sub-loop 105 which is differentiated by the differentiator 107 are added together and are outputted by an adder 109. Thus, the multi stage noise shaping quantizer is implemented of which noise shaping order is equal to “the noise shaping order of the sub-loop 105 plus 1.” In this example, the feedback circuit 131 may include a transfer function by which the sub-loop 105 indicates its noise shaping order higher than or equal to 2. The output of the feedback circuit 131 of the sub-loop 105 is inputted to the adder 113 of the main loop 103. Thus, the output of the feedback circuit 131 is added to an input to the first local quantizer (the output of the adder 111). As mentioned above, the multi stage noise shaping quantizer 101 can suppress a magnitude of a quantization error which is to be inputted to the sub-loop 105 according to the output of the feedback circuit 131 and maintains the operation of the sub-loop 105 in a stable state. In such manner, the first local quantizer 117 performs quantizing operation to the input which is formed by the addition of the output of the feedback circuit 131 to the output of the adder 111 and outputs the resultant to the limiter 141. The limiter 141 references a condition of the differentiator 107 and limits the value of the signal to be outputted from the limiter 141 based on the condition of the differentiator 107.
At a clock when the limiter 141 does not work, the magnitude of the quantization error occurred in the main loop 103 can fall within the range of values of signals inputted to the first local quantizer 117. On the contrary, at a clock when the limiter 141 works, the magnitude of the quantization error occurred in the main loop 103 can become larger than that which might be occurred in the case where the limiter 141 did not work at that clock. Therefore, there may be the case where the value of the signal inputted to the first local quantizer 117 may grow greater at clocks after the clock when the limiter 141 works, and as a consequence, the first local quantizer 117 may overflow.
FIG. 2 shows a relationship between a value range which may be inputted to the first local quantizer 117 and an actual signal example inputted to the first local quantizer 117. With reference to this figure, the occurrence of the overflow at the first local quantizer 117 of the multi stage noise shaping quantizer 101 in FIG. 1 is described. The input range 151 being defined as a range of value which can be inputted to the first local quantizer 117 may include an input signal part 157 such as an audio signal, a quantization error part 155, a feedback output part 153 which is fed from the sub-loop 105, and a margin part 191. The margin part 191 is for the case of a limit error occurred from operation of the limiter 141. On the other hand, the example of signal 159 which may actually be inputted to the first local quantizer 117 may include the input signal part 157, the quantization error part 155, the feedback output part 153, and a limit error part 195. As illustrated, in the multi stage noise shaping quantizer 101, the magnitude of the signal 159 which is actually inputted to the first local quantizer 117 sometimes exceeds the input rage 151 of the first local quantizer 117 so that an overflowed portion 193 may occur. Needless to say, the overflowed portion 193 is also fed back after a clock. This overflowed portion 193 being fed back will blow over while one or more clocks go by.
It is to be understood that the larger the margin part 191 is set, the more decreased in frequency the occurrence of the overflow is. At the same time, the larger the margin part 191 is set, the smaller the other parts 153, 155, and 157 have to be set. This means narrowing of the range which can be assigned to the input signal part 157. Therefore, this leads to reduce the dynamic range of the first quantizer 117.
Patent Reference 1: Japanese patent gazette No. 8-2024-B2
Non-Patent Reference 1: “A 17-bit oversampling D-to-A conversion technology using multistage noise shaping”, Matsuya, Yasuyuki et al., IEEE Journal of Solid State Circuit, August, 1989, Vol. 24, No. 4, p. 969-975